DHTML moving Image question

I am tying to move an image to right direction very slowly and then return it back to left also very slowly.
W3Schools has a tutorial, but in the tutorial the movement happens very quickly. The W3Schools url that I am talking about is:

The code is at below, slightly modified by me to show the movement, but the movement happens very fast. How to do the movement slowly so that the image moves to right direction very slowly then comes back to left very slowly.

thank you for your response, and the image moves very slowly. Please note that the example does not work at Firefox but works at IE, Safari, Opera, and Chrome. I guess that the above example does not work at Firefox as the hspace attribute got deprecated....[http://www.w3schools.com/TAGS/att_img_hspace.asp]

Therefore, would you please see this code of W3schools.com. The example is at:

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